1. Field of the Invention
The present invention relates to a battery pack suitably used in an electric vehicle, such as hybrid electric vehicle (HEV), pure electric vehicle (PEV), or the like, which uses an electric motor as at least a part of its power source. Specifically, the present invention relates to a battery pack in which a pair of battery modules connected to each other in parallel, with each battery module including a plurality of battery cells connected in series.
2. Description of the Related Art
An electric vehicle, such as HEV, PEV, or the like, uses a high-voltage battery for powering its electric motor. Usually, in order to gain a high voltage, such a high-voltage battery includes a plurality of battery modules connected in series or in parallel, and each of the battery modules includes a plurality of battery cells connected in series.
Generally, in designing such a battery pack, a positional arrangement of the battery modules is determined such that high-voltage wires for connecting the battery modules may be efficiently arranged.
A battery pack including battery modules connected in parallel has a characteristic of the size of a charging current flowing into each battery module depending only on a voltage of the battery modules. The voltage of each battery module depends on charging conditions, temperature, etc., of each battery cell.
The problems caused by such a characteristic are described with reference to FIGS. 5-7B.
FIG. 5 shows an equivalent circuit of a battery pack including battery modules connected in parallel. Referring to FIG. 5, a charging current I flowing through the entire battery pack is divided such that the voltages V1-Vn of battery modules 21-2n have the same value, and the divided currents flow through the battery modules 21-2n as battery currents I1-In. Electromotive forces E1-En and internal resistances R1-Rn of the battery modules 21-2n depend on conditions of battery cells in the battery modules 21-2n, such as temperature, state of charge (hereinafter, referred to as SOC), etc. As shown in FIG. 6, in general, when charging an NiMH (nickel metal hydride) battery, the voltage of the battery gradually increases until the SOC reaches a full-charge region, but after the SOC has reached the full-charge region, the battery voltage gradually decreases.
In such a NiMH battery including battery modules connected in parallel, in the case where the amount of charged electric power is different among the battery modules when the SOC of one of the battery modules reaches the full-charge region, the battery voltage of the battery module decreases, and accordingly, a charging current flows into the battery module. For example, in FIG. 7A, a pair of battery modules 21 and 22 connected in parallel are shown. When the battery module 21 is fully charged and the battery voltage V1 decreases, a charging current flows into the battery module 21 so that the battery voltage V1 has the same value as a battery voltage V2 of the other battery module 22. In FIG. 7B, a case where there is no charging current flowing from outside is shown. Even in this case, when the battery module 21 is fully charged, a current flows from the other battery module 22 which is not fully charged into the battery module 21. As a result, the battery module 21 is overcharged. When the overcharging of the battery module continues, battery cells in the battery module deteriorate. Such a problem may occur due to variation in the amount of electric power charged in battery cells.
According to one aspect of this invention, there is provided a battery pack including at least one pair of battery modules, each of the battery modules including a plurality of battery cells connected in series, wherein the at least one pair of the battery modules are connected in parallel and are positioned in close vicinity to each other such that radiant heat can be transferred between the pair battery modules.
According to another aspect of this invention, there is provided a battery pack including at least one pair of battery modules, each of the battery modules including a plurality of battery cells connected in series, wherein the at least one pair of the battery modules are connected in parallel and are coupled to each other by a heat transfer member.
According to still another aspect of this invention, there is provided a battery pack including at least one pair of battery modules, each of the battery modules including a plurality of battery cells connected in series, wherein the at least one pair of the battery modules are connected in parallel and are positioned in a region of a case which has a uniform cooling condition.
According to still another aspect of this invention, there is provided a battery pack including at least one pair of battery modules located in a case, each of the battery modules including a plurality of battery cells connected in series, wherein the at least one pair of the battery modules are connected in parallel and are positioned so as to obtain the same cooling condition from a cooling medium flowing through the case.
Thus, the invention described herein makes possible the advantages of providing a battery pack including battery modules connected in parallel, wherein differences in the state of charge between the battery modules is reduced by increasing differences in temperatures between battery modules.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.